A significant factor in the startup setup delay of calls is the transmission time of the signaling packets. An invite packet, such as a push-to-talk (“PTT”) session initiation protocol (“SIP”) invite packet, is generally used to initiate a dispatch call, and an OK packet, such as a 200 OK SIP packet, is sent in response to the invite packet. These packets may be 300 bytes, but in some cases are much larger. In many systems, this means that the transmission time of these packets will take 300 milliseconds or longer.
As typically done, in the startup of an invite packet transmission is that the transmission of the invite packet does not begin until the user has pressed the send or PTT button, but this transmission time of the invite packet can be very significant. This, in turn, creates additional delay because the transmitter must transmit the entire invite packet after receiving the button push.
Also, as is typically done, in the startup of a OK packet transmission is that in some systems, the transmission of the OK packet does not begin until the invite packet is received, but this transmission time of the OK packet can be very significant. This, in turn, creates another additional delay because the transmitter must transmit the entire OK packet after receiving the invite packet.
Data are generally broken down into data frames packaged into a single data packet at a time for transmission via a network. In most current conventional systems of dispatch and voice over Internet protocol (“VoIP”) calls, five data frames are typically sent per packet in order to decrease cost incurred by the per packet overhead. In order to wait for enough data to generate the data frames needed for a single data packet, however, the system must wait for approximately 120 millisecond (“msecs”) or more of audio to occur before transmission can begin. In fact, it may even be longer because the packet header is not sent until the entire packet is constructed. The transmission itself, then, typically takes approximately another 120 msecs or more, assuming the transmission is on a fundamental channel in a nominally loaded system. That is a total minimum wait time of approximately 240 msecs in order to transmit a data packet.
Although the wireless link layer, such as the Radio Link Protocol (RLP), allows the packets to be sent in sections, each frame contains only a length field along with a data field, but any extra space in the frame is padded so that no other information can be stored in the packet. This protocol, however, may skip unknown portions of the packet, and as a result, will send the skipped portions along with the portions that were previously sent as a retransmission at a later time, which wastes bandwidth. Moreover, this method does not adequately address the delay problems and the unknown portions of the packets, which is quite common in existing cellular systems.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Also, common and well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.